1. Field of the Invention
The present invention relates to a technique suitable for a semiconductor device and a process for producing the same, in particular, a semiconductor device having a capacitor, for example, a dynamic random access memory (DRAM), and a process for producing the same.
2. Description of Related Art
It is known that for large scale integration of semiconductor devices or other purposes, a tantalum pentoxide film having a dielectric constant of several tens is adopted instead of a silicon oxide film (dielectric constant: about 4) or a silicon nitride film (dielectric constant: about 7), which has been conventionally used as a capacitor dielectric film (for example, JP-A No. 139288/1998).
As a process for producing a capacitor having the tantalum pentoxide dielectric film, there is known a process of forming a tantalum pentoxide film on a capacitor bottom electrode, heat-treating the film to be crystallized, and then forming a capacitor top electrode thereon. The reason why the tantalum pentoxide film is heat-treated is to use a characteristic of tantalum pentoxide that its dielectric constant becomes larger when it is crystallized, and obtain a capacitor having a large electric capacitance. However, it is known that in order to crystallize the tantalum pentoxide film sufficiently in this production process, it is necessary to conduct heat treatment at a high temperature of 750° C. or more (for example, JP-A No. 12796/2000). The temperature at which the heat treatment for crystallizing a dielectric film is conducted is referred to as the “crystallization temperature” hereinafter.
Capacitor structure will be described before description on problems resulting from the matter that heat treatment at 750° C. or more, which is a relatively high temperature, is required for crystallization.
A capacitor using a tantalum pentoxide film as a dielectric film is roughly classified into MIS (metal-insulator-semiconductor) structure, which uses a polycrystalline silicon film as a bottom electrode, and MIM (metal-insulator-metal) structure, which uses a metal film as a bottom electrode. Differences between the MIS structure and the MIM structure are the following: 1) Their bottom electrode materials are different. 2) A barrier metal is necessary for the MIM structure. The barrier metal is formed between the bottom electrode and a plug connected to the bottom electrode, and is necessary for preventing reaction between the bottom electrode and the plug. If the plug reacts with the bottom electrode, a bad effect is produced on electrical conductivity. An example of the barrier metal is titanium nitride formed between a plug made of polycrystalline silicon and a bottom electrode made of ruthenium.
The following will describe problems resulting from the matter that heat treatment at 750° C. or more, which is a relatively high temperature, is required for crystallization in a process for forming a tantalum pentoxide insulator film. The MIS structure is heat-treated, thereby oxidizing silicon of its bottom electrode. As a result, the capacitance thereof drops. The reason for the drop is that since silicon is more easily subjected to thermodynamic oxidation than tantalum, silicon reduces the tantalum pentoxide film during heat treatment for crystallization of tantalum pentoxide so that a silicon oxide film, which has a small dielectric constant, is formed. The problem of the drop in the capacitor capacitance is also generated in the case of forming a silicon nitride film on the surface of the bottom electrode to prevent oxidation of silicon. Because the silicon nitride film is oxidized for the same reason so that the capacitor capacitance drops. In the MIM structure, its barrier metal is oxidized by oxygen diffusing in its metal electrode even if the metal electrode itself does not undergo any problem of oxidation. As a result, the electrical conductivity thereof is damaged. The reason for the damage is as follows: For example, in the case that the bottom electrode is made of ruthenium, oxygen atoms easily diffuse through ruthenium so that oxygen accumulates in the bottom electrode in the step in which the tantalum pentoxide film is formed; therefore, the barrier metal is oxidized by the accumulating oxygen in a subsequent step of heat treatment for crystallization of the dielectric film.
The respective problems peculiar to the MIS structure and the MIM structure do not depend on atmosphere at the time of the heat treatment for crystallization. When a capacitor is formed using a tantalum pentoxide film, heat treatment may be conducted in oxygen atmosphere. Even if the oxidation temperature in the heat treatment is made low so that the oxidation of the bottom electrode and the barrier metal can be suppressed, the problems are not fundamentally solved if the crystallization temperature cannot be made low.
As far as tantalum pentoxide is used as the capacitor dielectric film, it is very difficult to make the temperature for crystallizing tantalum pentoxide as low as 750° C. or less. Examples of the heat treatment in oxygen atmosphere include heat treatment performed in oxidation atmosphere to repair oxygen vacancy in the capacitor dielectric film, and heat treatment performed in oxidation atmosphere to remove residual carbon, which causes leakage current in the capacitor dielectric film formed by chemical vapor deposition (CVD) or the like.
Thus, in order to solve the problems based on a relatively high heat treatment temperature for crystallizing the tantalum pentoxide insulator film, the present inventors added niobium pentoxide to tantalum pentoxide and examined change in various properties.
First, FIG. 10 shows experimental results about a MIM structure. As a sample, there was used a film made of a composition tantalum pentoxide and niobium pentoxide and formed on a structure of Pt (200 nm)/Ti (10 nm)/SiO2 (100 nm) by sputtering. To form the film, a mixed gas of N2 and O2 (pressure ratio between N2 and O2:1/1) having a pressure of 10 mTorr was used. The substance temperature was 300° C., and the film thickness was 20 nm. After the formation of the insulator film, heat treatment was conducted within the temperature range of 500 to 800° C. in nitrogen gas flow for 1 minute. Thereafter, heat treatment was conducted at a temperature of 500° C. in oxygen gas flow for 2 minutes. The temperatures for crystallizing a solid solution of tantalum pentoxide and niobium pentoxide formed under the above-mentioned conditions and the dielectric constants thereof after the crystallization were compared in the case that the ratio of Nb was 0%, 10%, 50%, 90% and 100%, respectively. The results are shown in FIG. 10. The transverse axis thereof represents the Nb content, and the vertical axes thereof represent the crystallization temperature and the dielectric constant. In the case that the Nb ratio was 0%, that is, in the case of the film made only of tantalum pentoxide, the crystallization temperature was about 750° C. and the dielectric constant was about 30. As the Nb content was increased, the crystallization temperature lowered and simultaneously the dielectric constant increased. In the case that the ratio of Nb was 100%, that is, in the case of the film made only of niobium pentoxide, the crystallization temperature was about 500° C. and the dielectric constant was about 60. In order to set the crystallization temperature to 700° C. or less, at which oxidation of the bottom electrode and the barrier metal can be suppressed up to such a degree that no problem is caused, it is advisable that Nb is added at a ratio of at least 10%
Experimental results about a MIS structure are shown in FIG. 11. The temperatures for crystallizing a solid solution of tantalum pentoxide and niobium pentoxide formed on silicon and the dielectric constants thereof after the crystallization were compared in the case that the ratio of Nb was 0%, 10%, 50%, 90% and 100%, respectively. The results are shown in FIG. 11. The transverse axis thereof represents the Nb content, and the vertical axes thereof represent the crystallization temperature and the dielectric constant. In the case that the Nb ratio was 0%, that is, in the case of the film made only of tantalum pentoxide, the crystallization temperature was about 750° C. and the dielectric constant was about 40. In order to set the crystallization temperature to 700° C. or less, at which oxidation of the bottom electrode can be suppressed up to such a degree that no problem is caused, it is advisable that Nb is added at a ratio of 60% or more. As the Nb content was increased, the crystallization temperature lowered and simultaneously the dielectric constant increased. This tendency is the same as in FIG. 10. However, FIG. 11 is different from FIG. 10 in that at an Nb ratio of 50%, the crystallization temperature rises up to about 750° C.
FIG. 12 shows results of comparison of leakage current densities of insulator films having different Nb ratios. The transverse axis represents voltage, and the vertical axis represents the leakage current density. The heat treatment temperature was 700° C. As the Nb content was increased, the leakage current density increased.
As described above, in order to avoid a relatively high heat treatment temperature when a tantalum pentoxide insulator film is adopted as the dielectric film, it is effective to use a film to which niobium pentoxide is added or a film made only of niobium pentoxide. As understood from FIG. 12, however, the inventors found out a problem that when niobium pentoxide is added, leakage current density increases.